1. Field of the Invention
This invention relates to a multiple-color image output apparatus such as a color laser-beam printer, as well as to a multiple-color image output method.
2. Description of the Related Art
Image output apparatuses using electrophotography, such as laser-beam printers, have come to be widely used as output apparatus for computers. These image output apparatuses have been a primary factor in the rapid growth of the desktop publishing field owing to such features as high printing quality, quiet operation and high speed.
Furthermore, a recent trend has been to improve image quality by incorporating such quality-improving techniques as smoothing processing for detecting and smoothing the edges of characters and figures. Printer engines, which serve as the printing mechanisms in printers and have a high resolution of 600 dpi or better, have appeared in place of 240-dpi engines that used to be the standard. By combining the above-mentioned smoothing processing techniques with this higher resolution, these devices provide much better printing quality.
Electrophotographic color printers have also been eveloped. Owing to the improved performance of host computers and controllers for generating the images in printers, color images can be accommodated and printed in addition to conventional monochromatic images. Such color printers have been put to practical use and are becoming increasingly popular. Methods of printing full-color images exhibiting tonality by such color printers include the dither method, density pattern method and error-diffusion method. All of these methods are so-called pseudo-halftone methods which express tones based upon the ratio between printed dots and unprinted dots in a prescribed area. Furthermore, in laser-beam printers in particular, a characterizing feature is that resolution in the main-scan (horizontal scanning) direction can be changed with comparative ease. For example, such printers employ pulse-width modulation which expresses density by varying the driving pulse width of a laser diode in dependence upon the level of image data. This pulse-width modulation technique is superior to pseudo-halftone techniques, which are typified by the dither method, in that it affords both excellent tonality and high resolution.
A method of forming a color image in such a color-image output apparatus includes repeating, a plurality of times, a process through which a recorded image formed on an image bearing body by corona discharge, exposure and development is transferred to recording paper. This provides a color image by forming images of a plurality of overlapping colors on the recording paper. This method is implemented by an arrangement described in the specification of Japanese Laid-Open Patent Application No. 50-50935.
A photosensitive drum is used as the image bearing body in electrophotography. An electrostatic latent image that has been formed on the photosensitive drum is developed by affixing toner to the image. One example of a developing method is a single-component method that makes it possible to employ a process cartridge scheme which does not require complex elements such as an ATR or screw and which facilitates user maintenance. Non-contact development, which is one of the single-component development techniques, is particularly advantageous owing to the simple arrangement involved. Specifically, with contact development, the developing roller and the photosensitive drum contact each other, meaning that one of them must be a resilient body. With the non-contact development method, however, rigid bodies such as aluminum substrates, for example, can be used for these members. This is advantageous in terms of cost.
Further, for each color, it is preferable to use toner which melts instantly at a fixing temperature and is mixed with other color toner for better expression of colors of an output image. However, such toner has a characteristic of low temperature of glass transition in general. When such toner is utilized in the contact development method, the toner is possibly fused and fixed to the photosensitive drum and/or the development roller. To prevent toner from being fused and fixed to the photosensitive drum and/or the development roller, the non-contact method is preferable in electrophotography.
Though the non-contact development method has many advantages, the inventors have found that when a color image in which different colors neighbor each other in the manner shown in FIG. 17A is formed using this method, white gaps or spaces that should not be present appear between the neighboring colors in the image formed, as depicted in FIG. 17B. The reason for this phenomenon is that when a latent image which has been formed on the photosensitive drum and exhibits a sudden change in drum surface potential, as is the case with the image of an edge, is developed by the developing unit, there are occasions where the toner image is formed more narrowly than the electrostatic latent image originally formed on the photosensitive drum. In the case of monochromatic image formation, there are no neighboring colors and, hence, no problems arise even if there is some narrowing of the toner image. However, when a color image is formed under these conditions, as when an image having neighboring bands of cyan and black is formed, as shown in FIG. 17B, unwanted spaces appear between the neighboring cyan and black bands.
This narrowing of the toner image is a phenomenon that occurs because an electric field becomes involved, as shown in FIG. 18, at the edges of the electrostatic latent image formed on the photosensitive drum. The influence of this phenomenon on the image becomes pronounced in non-contact development. A method available to mitigate this phenomenon involving the electric field entails lowering the charging potential when the surface of the drum is uniformly charged. Though this method has the effect of reducing the narrowing of the toner image, a so-called fogging phenomenon, in which toner becomes affixed to the non-print areas, occurs, and the potential difference between print and non-print areas diminishes. The result is that a satisfactory image density cannot be obtained.